Active matrix substrates that are used for liquid crystal display devices or the like include a switching element for each pixel, e.g., a thin film transistor (hereinafter “TFT”). As such switching elements, TFTs whose active layers are an amorphous silicon film (hereinafter “amorphous silicon TFTs”) and TFTs whose active layers are a polycrystalline silicon film (hereinafter “polycrystalline silicon TFTs”) have been widely used.
In the recent years, it has been proposed to use an oxide semiconductor as the material of the active layers of TFTs, instead of an amorphous silicon or a polycrystalline silicon. These TFTs are called “oxide semiconductor TFTs”. An oxide semiconductor provides a higher mobility than does an amorphous silicon. Therefore, oxide semiconductor TFTs can operate more rapidly than amorphous silicon TFTs. Moreover, an oxide semiconductor film is formed through a simple process as compared to a polycrystalline silicon film, and therefore is applicable to devices which require a large geometric area.
Patent Documents 1 and 2 disclose TFTs of a bottom gate structure in which an oxide semiconductor is used. In the TFT disclosed in Patent Document 1, metal oxide layers are formed between an oxide semiconductor layer and source/drain electrodes for enhanced contact between them. Patent Document 2 proposes allowing an oxide semiconductor layer to react with source/drain electrodes, thus forming reaction layers therebetween.